Electric Water Heater: Complete Wiring Guide
Bill of materials
Tank-style electric water heaters typically draw 4,500W on a 30A/240V circuit using 10 AWG wire. Tankless units draw far more: a whole-house 24kW unit requires 100A total, often split across two or three 40A circuits. NEC 2023 added GFCI requirements for outlet-connected water heaters.
Tank water heaters: 30A circuit with 10 AWG copper, hardwired with a disconnect within sight per 422.31. Tankless: check manufacturer specs carefully. A 24kW unit draws 100A at 240V, typically requiring three 40A double-pole breakers with 8 AWG to each element bank. NEC 2023 Section 210.8(D) requires GFCI for cord-and-plug connected water heaters. Hardwired units are exempt but some jurisdictions interpret the requirement more broadly.
Tank water heater (4500W standard)
| Component | Specification | NEC basis |
|---|---|---|
| Wire (copper) | 10 AWG THHN | Table 310.16, 110.14(C) |
| Wire (aluminum alt.) | 8 AWG THHN | Table 310.16, 110.14(C) |
| Breaker | 30A double-pole | 240.4 |
| Voltage | 240V single-phase | |
| Conduit (EMT) | 1/2" EMT | Chapter 9 Table 4 |
| GFCI | Required (GFCI breaker or device) | 210.8(D) |
| Max distance (3% VD) | 153 ft at 240V copper | 210.19(A) Note 4 |
Tankless water heater (24kW), each of 3 circuits
| Component | Specification | NEC basis |
|---|---|---|
| Wire (copper) | 8 AWG THHN | Table 310.16, 110.14(C) |
| Wire (aluminum alt.) | 6 AWG THHN | Table 310.16, 110.14(C) |
| Breaker | 40A double-pole | 240.4 |
| Voltage | 240V single-phase | |
| Conduit (EMT) | 3/4" EMT | Chapter 9 Table 4 |
| GFCI | Required (GFCI breaker or device) | 210.8(D) |
| Max distance (3% VD) | 138 ft at 240V copper | 210.19(A) Note 4 |
Installation notes
This project requires GFCI protection per NEC 210.8(D). The simplest approach is a GFCI breaker at the panel, which protects the entire circuit. For 240V circuits, use a 2-pole GFCI breaker sized to match the circuit. GFCI breakers cost $40-80 depending on amperage.
For the wire run, you have two options: NM-B cable (Romex) for interior runs through framing, or individual THHN conductors in conduit for exposed runs, outdoor installations, or runs through unfinished spaces. NM-B is faster to install but cannot be used outdoors or in conduit. For conduit, use EMT (metallic) or PVC Schedule 40.
Run length matters: every foot of wire adds voltage drop, reducing power to your equipment. The table below shows the exact voltage drop at common distances. If your run exceeds the 3% threshold, upsize one gauge.
Voltage drop by distance
Percentage voltage drop at common run lengths. NEC recommends 3% maximum for branch circuits and 5% for combined feeder plus branch.
| Distance | 10 AWG Co | 8 AWG Al | 8 AWG Co |
|---|---|---|---|
| 25 ft | 0.5% | 0.5% | 0.3% |
| 50 ft | 1.0% | 1.0% | 0.6% |
| 75 ft | 1.5% | 1.5% | 0.9% |
| 100 ft | 2.0% | 2.0% | 1.2% |
| 150 ft | 2.9% | 3.0% | 1.9% |
| 200 ft | 3.9% | 4.0% | 2.5% |
Common products for 10 AWG Copper circuits
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Related wire data
Detailed ampacity: 10 AWG copper, 8 AWG aluminum. Wire size lookup: 30A circuit. Calculators: voltage drop · panel load calculator.
Frequently asked questions
What wire size do I need for electric water heater?
Use 10 AWG copper or 8 AWG aluminum on a 30A breaker at 240V. This is based on NEC 2023 Table 310.16 ampacity with 110.14(C) termination limits. The equipment nameplate or installation manual sets the minimum circuit ampacity and maximum breaker; follow it where it differs.
Does electric water heater require GFCI?
Yes. GFCI protection is required per NEC 210.8(D).
NEC 2023 references verified February 2026